The process variable is, for example, the flow rate of a fluid flowing through a measuring tube or pipe or the fill level of a medium in a container. However, also other process variables fall within the scope of the invention, such as, for example, the viscosity or the density of the medium. A wide variety of corresponding field devices are produced and sold by the company, Endress+Hauser.
The registering of the process variable of interest can be influenced by the most varied of parameters of the environment, which negatively influence the measurement result. An example is given by the influence of temperature, more exactly the temperature of the medium, or, in given cases, also the ambient temperature. Temperature plays a large role in many different field devices, and, thus, the most varied of measures are known for compensating its influence on the registering of a process variable by means of a field device. These measures are as varied as the different measuring principles.
For example, described in German patent, DE102010030791A1 is a circuit for compensating the influence of temperature in the case of a vibronic fill-level measuring device. The underpinning measuring principle is known from a large number of publications. An oscillatable unit is excited to execute mechanical oscillations by means of an electromechanical transducer unit driven by an electrical exciter signal. For eliminating the influence of temperature on the measuring, the electromechanical transducer unit is connected in parallel with a reference element, which is likewise supplied with the exciter signal. By means of an algorithm, the oscillation independent reference signal obtained with the reference element is taken into consideration with the measurement signal, so that the influence of temperature can be compensated.
A temperature compensation circuit is likewise often discussed in connection with ultrasonic, flow measuring devices. Also for this category of field devices, the underpinning measuring principles are known from a large number of publications. In the case of the travel-time difference principle, the different travel times of ultrasonic pulses in, and counter to, the flow direction of the medium are evaluated, while in the case of the Doppler principle an ultrasonic pulse of adjustable frequency is coupled into the medium and the reflected signal detected.
For eliminating the influence of temperature in the case of a piezooscillator, the German Patent, DE19820208A1 describes the provision of a temperature dependent component in the form of a temperature dependent resistance connected in parallel. In such case, both the flow rate and the temperature are transmitted via the same signal line. Therefore, disadvantageously in the case of such a circuit arrangement, the temperature measurement can be negatively influenced by an additional warming of the temperature dependent resistance.
In German Patent, DE10057188B4, an energetic decoupling of the temperature sensor from the ultrasonic signal emitter can be achieved by the provision of a coil. However, in the case of a coil, as a result of self-warming, fluctuations of resistance and/or inductance can occur, which likewise have a negative influence on the determining of temperature.
German Patent, DE102013100670A1 discloses an ultrasonic, flow measuring device with a temperature compensation, which is performed by a temperature sensor connected in parallel with a piezoelement. The temperature sensor includes a temperature dependent measuring element, a limiting resistor for power limiting and/or a limiting circuit, and the temperature is determined in measuring pauses of the ultrasound flow measurement and/or during the transmitting of an ultrasonic transducer in the measurement circuit.
In the case of all these compensation circuits for ultrasonic, flow measuring devices, the measurement signals for flow measurement and for temperature compensation are transmitted via the same line. Furthermore, it is necessary that within an electronics unit, in which the measurement signals are processed as well as the flow rate determined, there be a switching between a module for the flow measurement and an additional module for the temperature measurement. This is, on the one hand, a comparatively complex solution, and, on the other hand, it cannot be prevented that in the course of the signal transmission by means of the same line a mutual influencing of the temperature- and flow measurements occurs.
Besides the temperature, parameters of the environment can include, for example, also the expansion of a tube, or pipe, wall as a result of pressure of a flowing medium. Known in the state of the art for this influence are approaches similar to those used for ultrasonic, flow measuring devices, in order to determine the influence on the measuring of the flow, which approaches have correspondingly also similar disadvantages.
It would, however, be desirable, to determine a parameter of the environment for a measured variable in such a manner that the determination is not influenced by the actual measuring.